There's no downside to adding any character from any franchise.
I'm not sure your point.
Well I will say that the characters that are made need to make Profit compared to how much work goes into the inclusion of every fighter
Granted, I don't think there are too many Fighters discussed that I think wouldn't make Profit anyway. but if a Character was usper niche, while also being very complicated to Implement, that might be a downside to Implementing them since The reward for the efforts doesn't pay back the amount of work that was needed
What I mean Downside, I mean Obstacles in terms of why Nintendo wouldn't choose a Certain character, or can't Implement a character
As an example, With Sora, there's the obstacle of Disney making things complicated. Sure a represented of Disney said they were down for Sora in Smash, but when it comes of the actual negotiations, Some details of what Disney may want might conflict with what Nintendo wants, and that also doesn't go into the pricing of things and stuff like that. We don't have confirmed info, but only Rumors from some inside people that suggests negotiations didn't go as planned when it came to Kingdom Hearts in Smash
This doesn't mean that Sora has no chance at Smash ever, or anything like that. what it does mean is that there are some obstacles in terms of whether Sora would be prioritized to be one of the next Fighters
There's characters that have Regional based popularity, where they popular in one region, not so much in another. People don't pay attention to Mortal Kombat in Smash, despite how huge the Franchise is, because of the Whole Japan thing. People are wary of Reimu because of her Relative lack of popularity in the West
There's all these factors that people take into Account when speculating characters, because these are probably factors Nintendo takes into account when deciding characters
Another Sonic character Doesn't really have any Obstacles in why they can't be in. the biggest one I could think of is that it's not as Popular in Japan as it is everywhere else. even then it has had Good amount of popularity in the past and still does have a great fanbase there. I've also Seen Shadow being on some Japanese based polls in the past as well
Characters like Tails, Shadow, & Knuckles are characters I've seen people want for years and years, even decades. They are Very well known even to Casual Players, They are relevant characters, and Sega loves licensing out the Sonic Brand, Plus Nintendo has already worked with Sega to implement in Sonic for 3 games, along with Having Bayonetta and Joker along for the ride.
That's what I mean when there's no downside, because there's nothing that's in the way of us getting another Sonic character, besides there being other potential options, and that's where it starts being speculative and based on your own interpretation of what Nintendo might choose
This is part of the Reason that Crash is so prevalent on many people's Prediction list, because from the surface there is no Downside to adding him to Smash. this is also why you don't see as many predictions on characters like Reimu, Falcom Characters, or stuff like that, because people do find they have their drawbacks in terms of consideration
We see with Sephiroth that they are not afraid to continue adding more Content from a Popular Game or Franchise (FF7), so I ask why they couldn't do the same with Sonic. If there's no drawback to adding in Another Sonic character, and you find that ALOT of people want another Sonic character, and have for a long while, what's the hold up at that point.